1. Field of Use
This invention relates generally to an electro-hydraulic control system for a multi-section hydraulically operated telescopic crane boom. In particular, it relates to such a control system which enables either simultaneous extension or retraction of all movable boom sections or independent extension or retraction of one or more selected movable boom sections.
2. Description of the Prior Art
The prior art contains many examples of telescopic crane booms and control systems for the hydraulic cylinders thereof whereby the movable boom sections can be extended or retracted. In some cases, the control system employed a separate independently operable hydraulic control valve for the hydraulic cylinder of each movable boom section. However, although the boom sections could be moved simultaneously or independently, separate fluid supply and return hoses were required for each control valve and its associated cylinder. This resulted in costly redundancy of valves, hoses, and other components and also created problems and expense in regard to the disposition, arrangement, operation, and servicing of many such hoses located within and between boom sections. In other cases, the control system employed a single control valve and a flow divider for feeding hydraulic control fluid to each boom cylinder. In such cases, only simultaneous movement of all boom sections was possible and "random telescoping" tended to result. This caused unequal stressing and loading of the various boom sections, which is an unacceptable and unsafe condition in some load-handling situations. The following U.S. patents are but a few examples of prior art telescopic boom control systems: 3,976,097; 3,666,125; 3,672,159; 3,624,979.